In recent years, as a wireless communication system that realizes high-speed transmission in a limited frequency band, a multi-user MIMO (MU (Multi-User)-MIMO) system obtained by applying a space division multiple access (SDMA) scheme to an MIMO system in which pluralities of antennas are set in both of a transmitter and a receiver has been actively examined. In the MU-MIMO system, a plurality of terminals including pluralities of antennas are present with respect to a base station including a plurality of antennas. The base station performs simultaneous transmission to the terminals in the same wireless frequency band.
In communication in a downlink, that is, a direction from the base station to the terminal in the MU-MIMO system, signals are simultaneously transmitted from the base station to the terminals. Therefore, in general, in a received signal in the terminal, signals to the other terminals are included in addition to a desired signal, which is a signal to the own terminal. That is, inter-user interference (IUI), which is interference caused by the signals to the other terminals occurs. IUI measures are desirably performed on the base station side, where limitations concerning a processing amount and the number of antennas are small compared with the terminals, as much as possible. Therefore, in the downlink in the MU-MIMO system, the base station carries out processing called precoding as the IUI measures. The precoding indicates processing for forming a beam by weighting a plurality of signals transmitted from a plurality of antennas.
As a representative precoding method performed as the IUI measures in the downlink in the MU-MIMO system, block diagonalization (BD) method has been extensively examined. Please refer to, for example, Non Patent Literatures 1 and 2. The BD method is a precoding method for forming a beam space to direct null to terminals other than a desired terminal, that is, form directivity for reducing received power in the terminals other than the desired terminal to 0. By applying the BD method to all the terminals, it is possible to realize a MU-MIMO system in which the IUI does not occur. Consequently, it is possible to simplify processing and an apparatus configuration in the terminals.